In order to obtain carbon-based thin films having high quality crystalline properties, it is essential to eliminate problems caused by the mismatch between the lattice constant of the thin film and that of the substrate material. In order to reduce the problem of lattice mismatch in the prior art, proper choice of the substrate onto which the films are deposited and appropriate surface processing of the substrate prior to deposition have been very important considerations in the development of high quality thin films.
By way of example, high quality diamond films have been deposited in the prior art onto a single crystal diamond substrate. This type of deposition, wherein the thin film and the substrate material are identical, is an example of "homo-epitaxial" film growth. Film deposition in which the substrate material is different from that of the depositing thin film material is correspondingly termed "hetero-epitaxial" film growth. The development of hetero-epitaxial growth techniques that are not limited by the choice of substrate and substrate processing requirements is a subject that is currently receiving a great amount of attention.
By using the techniques disclosed herein, crystalline carbon-based thin films can be deposited on a variety of different and readily available substrates, thereby expanding the useful range of applications for crystalline carbon-based thin films. In particular, the hetero-epitaxial growth of crystalline carbon-based thin films such as diamond on a silicon substrate is considered to be highly desirable because of the availability of high quality, large area silicon substrates and the applicability of crystalline carbon thin films grown on silicon substrates to the fabrication of hybrid semiconductor device structures. In addition to diamond thin films, silicon carbide thin films are another example of a crystalline carbon-based thin film system which is desirably deposited onto a silicon substrate by hetero-epitaxy. Development of hetero-epitaxial growth techniques is particularly important for crystalline carbon-based systems because the use of diamond or silicon carbide single crystal substrates is impractical. Such substrates are very expensive and are virtually impossible to obtain in large areas.
In the prior art, the hetero-epitaxial deposition of diamond or silicon carbide thin films generally has required pre-processing of the silicon substrate. For example, prior to the deposition of diamond thin films on a silicon substrate, the surface of the silicon substrate has been intentionally roughened in the prior art in order to enhance the number of nucleation sites on the substrate and thereby aid in the growth of the thin film. Roughening of the silicon substrate has been performed in the prior art by ion beam bombardment of the substrate with inert gas ions (e.g. argon) or by using mechanical abrasives.
In the case of silicon carbide thin film deposition, prior art methods have carbonized the surface of the silicon substrate prior to deposition. Other prior art methods have used specially prepared silicon substrates having crystallographic surface orientations that are offset by several degrees from the (100) crystallographic plane of silicon, so as to relieve the film stress which arises from the inherent lattice constant mismatch between the silicon substrate and the deposited silicon carbide thin film. Pre-processing of the substrate surface in the prior art has therefore been an important prerequisite to the deposition of high quality crystalline carbon-based thin films using hetero-epitaxial film growth.
Whereas the prior art has experimented with various methods to increase the nucleation density and to relieve lattice mismatch, these prior art methods have inherent problems. For example, while the selection of the surface orientation of the substrate material can play an important role in facilitating the matching of lattice constants between the substrate material and the deposited film, it is difficult and time consuming to control the crystal orientation of the surface of the substrate material and the high density of surface defects generated by the lattice mismatch is difficult to remove.
Thus, the presently available substrate materials and surface processing techniques for depositing high quality crystalline carbon-based thin films are inadequate. Particularly in the context of hetero-epitaxial growth, methods for fully solving the fundamental lattice mismatch problem are not available.